Three-dimensional MT Inversion For Arbitrarily Anisotropic Earth

After more than 70 years of development of magnetotelluric methods(MT),many mature MT data processing and interpretation technologies have been formed.Because one-dimensional and two-dimensional inversions have a high degree of simplification of the model,they are not suitable for complex three-dimensional(3D)underground structure imaging.Therefore,in recent years,3D inversion has become a hot research topic in the area of MT and has achieved rapid development.At present,3D MT inversion technology has been widely used in the fields of geodynamics,geothermal resources investigation,oil and gas and mineral exploration,and has achieved good application results.With the continuous improvement of the demand for refined interpretation of geophysical exploration,geophysicists have found that the influence of the anisotropy of the earth on the MT data cannot be ignored,but the current mainstream 3D MT inversion assumes that the earth is isotropic,which cannot handle the MT data affected by anisotropy.Since anisotropy plays an important role in the study of the deep core-mantle structure of the earth and the development of oil and gas resources,it is imperative to develop a 3D MT inversion technology for arbitrarily anisotropic earth.In view of the above situation,this paper proposes to carry out research on the 3D anisotropy inversion method of MT data.The main research contents are as follows:1)First of all,this paper discretizes the earth into an unstructured tetrahedral grid,and uses a 3×3 tensor to describe the anisotropic conductivity in the forward modeling governing equation of MT,which can be simplified to three-axis conductivity and three rotation angles in the principal axis direction.Then,the vector finite element method based on unstructured grid is used for forward modeling and adjoint forward modeling to calculate the full impedance response and sensitivity of MT.The numerical simulation results show that the forward modeling accuracy meets the requirements of inversion;2)To make the inversion tend to be flexible in the recovery of anisotropic and isotropic structures,this paper adds an additional regularization to the objective function to control the degree of anisotropy of the inversion model,whose weight is controlled by the distribution function.In addition,in order to further suppress the multi-solution problem of anisotropic inversion,this paper adds upper and lower bound constraints to the 6 model parameters.Finally,in this paper,the gradient of the anisotropic inversion objective function is obtained by the adjoint forward modeling technique,and the limited memory quasi-Newton method is used to solve the inversion problem;3)In this paper,four representative numerical experiments are designed to test the resolvability and resolution of the 6 anisotropy parameters in the inversion.The results show that the inversion method proposed in this paper can recover the strike angle and the resistivity in the x and y directions well.Finally,the inversion method proposed in this paper is used to process and interpret the long-period MT data set from the USArray,revealing the anisotropic structure of the Cascadia region,which has good consistency with the previous results.To sum up,the 3D MT inversion for arbitrarily anisotropic earth proposed in this paper overcomes the limitations of traditional MT inversion,improves the ability of MT data processing and interpretation,and provides the ideas of anisotropic inversion for other electromagnetic methods.